1. Field of the Invention
This invention relates to a process and apparatus for producing ferro-magnesium.
During the 1940's it was discovered that the graphite contained in ordinary gray cast iron compositions could be altered from its normal flake form to a nodular form by the addition of relatively small quantities of magnesium. The basic process was disclosed in the Millis, et al. U.S. Pat. No. 2,485,760. Essentially, gray cast iron consists of a steel matrix whose continuity is disrupted by a myriad of graphite flakes dispersed throughout the matrix. These graphite flakes are particles of leaf or saucer shape, formed by the precipitation of excess carbon as graphite from the liquid during solidification. It was then discovered that a small amount (e.g., 0.05%) of magnesium dissolved in the liquid iron would cause the formation of spheroidal graphite instead of flake graphite from the melt. Although the phase amounts and analyses of ductile or nodular iron are the same as those for ferritic gray cast iron, the difference in the shape of the graphite results in twice the tensile strength and twenty times the ductility. The development of nodular iron, among other things, created a need for a magnesium-containing product suitable as an addition agent for the treatment of molten ferrous products.
2. Description of the Prior Art
It has long been known that magnesium was an effective desulfurizer and degasifer of non-ferrous metals but magnesium reacts violently with molten iron and is soluble in iron only to a small degree. Hence, in the production of nodular iron the magnesium has been introduced in various combined forms such as ferrosilicon magnesium (see U.S. Pat. Nos. 3,177,071, 3,290,142, 3,367,771 and 3,375,104), coke or charcoal impregnated with magnesium (see U.S. Pat. Nos. 3,321,304 and 3,598,572), combinations of rare earths and magnesium and alloys of magnesium and nickel or copper (see U.S. Pat. Nos. 3,030,205 and 3,544,312).
However, in all cases the addition of the magnesium or magnesium alloy has been accompanied by the evolution of smoke and flaring due to the reaction of magnesium with the heated atmosphere. Attempts have been made to plunge the magnesium addition agent beneath the surface of the liquid cast iron in the ladle by means of a plunging bell or a gaseous stream. See, for example, U.S. Pat. Nos. 2,869,857, 3,080,228, 3,157,492 and 3,285,739. Another approach has been to place the magnesium additive in the bottom of the ladle, cover it with scrap steel punchings and then tap the liquid iron into the ladle.
Although these mechanical devices represent an improvement, they are only partially successful and typically 50% or more of the magnesium is lost. Not only is the loss of the magnesium an economic disadvantage, but also the concomitant production of smoke, fume and flaring is unacceptable in view of the ever more stringent statutes and regulations relating to air quality.
Another approach to the problem is through the use of various alloys. However, alloys containing substantial quantities of silicon and magnesium are considerably less dense than iron and tend to float to the surface of the liquid iron bath. Moreover, the usual liquid iron temperatures are above the boiling point of magnesium at atmospheric pressure and therefore the magnesium vaporizes almost immediately. As a result, only a small amount of the added magnesium has an opportunity to dissolve in the liquid iron before the bubbles of vapor reach the surface of the iron melt and escape to the atmosphere. It is believed that these two effects, i.e., the low density of the silicon-magnesium additives and the low boiling point of magnesium are principally responsible for the low efficiencies of the magnesium additives as used heretofore.
In an attempt to avoid these problems, a number of processes have been suggested. These processes may be classified as follows: (1) processes using pure magnesium in the ladle under high pressure, (2) introduction of pure, solid magnesium through an opening located in the lower part of the treatment ladle, (3) processes where the vaporization of the magnesium is delayed by passage through an inert porous material, and (4) processes where the magnesium is diluted as an alloy or ferrous agglomerate. This classification was suggested in a recent article entitled "Simple and Economic Use of Pure Magnesium in the Production of Spheroidal Graphite (Ductile) Iron" by Henri Jarysta which appeared in the Proceedings of the International Magnesium Association for 1976 at pp. 49-52. Mr. Jarysta concluded that the above processes all had the disadvantages of either requiring sophisticated and expensive installation or resulting in excessive additive cost and recommended the use of small magnesium ingots coated with an isolating refractory.
Other researchers have directed their attention to the desulfurization of pig iron by the addition of magnesium and developed methods for the introduction of magnesium, magnesium alloys and magnesium-entrained coke into the bath similar to those systems developed for making nodular iron. Some of the mechanical systems involve (1) pressurized tubes through which magnesium ingots are submerged in the bath, (2) tilting reactors, (3) pneumatic injection of powdered or granular magnesium, (4) refractory-coated magnesium ingots and (5) wire injection. As shown in an article entitled "The Kinetics of Magnesium Vapour Dissolution Into Pig Iron" by Gordon A. Irons and R. L. Guthrie which appeared in the Proceedings of the International Magnesium Association for 1976 at pp. 63-72, this work has focused on the question of dissolution of vaporized magnesium into pig iron.